It is a fundamental principle, on which a polymer electrolyte membrane fuel cell is based, that a fuel gas containing hydrogen and an oxidant gas, such as oxygen, containing oxygen are electrochemically reacted whereby electric power and heat are generated at the same time, and that the thus generated electric power is taken out. This fuel cell basically comprises a polymer electrolyte membrane for selectively transporting hydrogen ions and a pair of electrodes, i.e. anode and cathode, formed on opposite surfaces thereof. The electrode is structured by a catalyst layer having, as a main component, a carbon powder carrying a platinum metal catalyst and by a gas diffusion layer formed on an outer surface of the catalyst layer and having both gas permeability and electronic conductivity.
Gas sealing members and gaskets to sandwich the polymer electrolyte membrane therebetween are placed at peripheral portions of the electrodes in order to prevent the supplied fuel gas and oxidant gas from leaking to outside and from mixing with each other. The sealing members and the gaskets are preliminarily and integrally assembled with the electrodes and the polymer electrolyte membrane. This is called MEA (electrolyte-electrode assembly). Electrically conductive separators are placed at outer sides of the MEA for mechanically fixing it and for electrically connecting, in series, neighboring MEAs with each other. On a portion of each separator plate to contact with the MEA, a gas flow channel is formed for supplying a reactive gas to the electrode and for carrying away a generated gas and excessive gas. The gas flow channel can be provided separately from the separator plate, but it is a general way to provide plural grooves, as gas communication grooves, on the front and rear main surfaces of the separator plate. In this connection, it is to be noted that bank-shaped portions between neighboring grooves on a same main surface thereof is referred to as ribs. One cell, namely unit cell, is structured by a pair of such separator plates and by an MEA sandwiched therebetween.
In order to supply gas communication grooves on one main surface out of the front and the rear main surfaces of a separator plate with a fuel gas and exhaust e.g. excessive gas therefrom, and to supply gas communication grooves on the other main surface thereof with an oxidant gas and similarly exhaust e.g. excessive gas therefrom, it is a general method to provide the separator plate with two through-holes and to respectively connect the inlet and outlet of the gas communication grooves with these through-holes, whereby each reactive gas is directly supplied, by being furcated, to each gas communication groove from one of the through-holes, while each reactive gas is exhausted from the other through-hole. The through-holes for supplying each reactive gas to each gas communication groove and for exhausting e.g. excessive gas from each gas communication groove are referred to as manifold openings. Such gas supply/exhaustion method is referred to as inner manifold system.
Other than the inner manifold system, there is another method, which is referred to as outer manifold system. The outer manifold system is such system that a pipe arrangement for supplying each reactive gas is furcated to the number of the separator plates to be used, and that each furcated portion is directly connected into each groove of the separator plate by using a piping jig which is referred to as manifold.
Further, the fuel cell is usually cooled by a cooling medium, because the cell generates heat during its operation. Usually, a cooling member for flowing a cooling medium is provided for every 1 to 3 cells. In this regards, it is an often employed way to assemble two separator plates each having, on one main surface thereof, gas communication grooves for a reactive gas and having, one the other main surface thereof, a flow channel for a cooling medium in a manner that the both other main surfaces, i.e. both surfaces each having the flow channel for the cooling medium, are contacted with each other to form a cooling member.
These MEAs, separator plates and, depending on needs, cooling members are alternately stacked to assemble a stack of 10 to 200 cells. Such stacked body is referred to as cell stack. The cell stack is sandwiched by end plates, with current collecting plates and insulating plates being present therebetween, and the cell stack is fixed by tightening both end plates, using tightening bolts, with a pressure being thereby applied to the cell stack, whereby a fuel cell having structure of a general cell stack system is formed.
It has been a common sense in such fuel cell according to prior art (for example, Japanese Laid-open Patent Publication 2000-133291) to form gas communication grooves, hence ribs, on one main surface out of a front main surface and a rear main surface of each separator plate are formed to be positioned in correspondence with gas flow channel (sic gas communication grooves), hence ribs, on the other main surface. And it has been a common sense to stack separators in such a manner that ribs, hence gas communication grooves, of all separator plates in a cell stack simply align with each other from one end to the other end of the cell stack, and that a tightening force for fixing the cell stack is transferred through the ribs.
However, in the case of such a fuel cell of prior art, bottom portions of the communication grooves between both gas communication grooves on both front and rear main surfaces of each separator plate are thinnest wall portions in the separator plate. Consequently, it has been very likely that cracks or fractures are generated at the groove bottom portions, resulting in gas leakage at such portions, by the pressure of the tightening bolts in the manufacturing of the cell stack or by pressure applied to the fuel cell during use of the fuel cell after the manufacturing. Conversely describing, under a strong requirement for thinner fuel cells in recent years, the conventional method of simply stacking separator plates having thinnest wall portions at the groove bottom portions of each of the separator plates has had a limitation of thinning the fuel cells because of the limitation of strength of the groove bottom portions.
Further, when separator plates are manufactured by compression molding or injection molding using a mold for the purpose of lowering cost, it is difficult for a material for separators to flow into portions of the mold corresponding to the above described thin wall portions of separator plates. Thus, there has also been a problem of difficulty of manufacturing.